This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application; entitled Mode Shape Converter, Method For Fabricating the Mode Shape Converter And Integrated Optical Device Using The Mode Shape Converter earlier filed in the Korean Industrial Property Office on Jan. 21 1999, and there duly assigned Serial No. 99-1764 by that Office.
1. Field of the Invention
The present invention relates to a mode shape converter, a method for fabricating the mode shape converter, and an integrated optical device using the mode shape converter, and more particularly to a mode shape converter arranged at an input or output terminal of an optical device and adapted to couple lights inputted into or outputted from the optical device via optical fibers. The present invention also relates to a method for fabricating such a mode shape converter, and an integrated optical device using the mode shape converter.
2. Description of the Prior Art
An integrated optical technique is a technique for integrating a variety of optical devices using waveguides on one substrate. Using such an integrated optical technique, it is possible to easily integrate a multi-functional optical device having a complicated structure on a limited small area because the alignment of unit optical devices can be easily made.
Examples, incorporated by reference herein, of waveguide structure implementing an integrated optical device are disclosed in U.S. Pat. No. 5142,596 to Kiminori Mizuuchi et al. entitled Tapered Light Wave guide And Wavelength Converting Element Using Tile Same; U.S. Pat. No. 5,391,869 to Robert W. Ade et al. entitled Single-Side Growth Reflection-Based Waveguide-Integrated Photodetector; U.S. Pat. No. 5,910,012 to Takeshi Takeuchi entitled Waveguide Type Semiconductor Photodetecting Device Method For Fabricating; and U.S. Pat. No. 5,078,516 to Elyahou Kapon et al. entitled Tapered Rib Waveguides.
A rib waveguide is a channel waveguide fabricated by partially etching a planar waveguide. Such rib waveguides have various advantages as follows. First, it is possible to select respective refractive indices of a core and a clad within a wide range. Second, it is possible to fabricate a single-mode waveguide having a large cross-sectional area irrespective of a refractive index difference between the core and clad. Third, it is possible to easily adjust optical characteristics such as a mode distribution and a propagation constants under the condition in which an etched depth is used as a major process parameter. Fourth, a precise pattern can be obtained, as compared to rectangular waveguides. This is because the etched depth in the rib waveguide is less than those of the rectangular waveguides. Fifth, it is possible to reduce damages occurring during an etching process for a core layer, for example, errors in pattern size caused by an anisotropic etching, a cracking phenomenon occurring during the etching process for a layer having stresses, and damages caused by a re-accumulation of by-products formed during the etching process.
In spite of such advantages, the above mentioned rib waveguide has a disadvantage in that a very large coupling loss is generated when an optical fiber is coupled to the waveguide of the optical device. Single-mode optical fibers have a circular mode distribution having an aspect ratio of 1:1 while having a relatively large size, for example, about 10 xcexcm. On the other hand, rib waveguides have an oval mode distribution in which its horizontal width is larger than its vertical width. In many cases, the mode distribution size of such a rib waveguide is also larger than those of the single mode optical fibers. For this reason, there is a misalignment in mode shape at the connection between a rib waveguide and an optical fiber. Due to such a mode shape misalignment, an optical wave encounters with a discontinuity while passing through the connection, so that it involves a coupling loss while being reflected or scattered. In order to solve this problem, a mode shape converter is arranged at the input or output terminal of the integrated optical device to which an optical fiber is coupled. The mode shape converter serves to conduct the function for slowly converting the mode of the optical fiber into a mode shape suitable for execution of the functions of the optical device, thereby achieving a reduction in coupling loss.
FIG. 1 is a perspective view illustrating the structure of a conventional mode shape converter disclosed in U.S. Pat. No. 5,078,516. She mode shape converter shown in FIG. 1 includes a first waveguide 100, a second waveguide 102, and a substrate 104. In FIG. 1, the reference numeral 106 denotes an input terminal whereas the reference numeral 108 denotes an output terminal. The reference numeral 110 represents respective refractive indices of the first waveguide 100, second waveguide 102, and substrate 104. The first waveguide 100 is designed to have a small mode size suitable for execution of the functions of an optical device to which the mode shape converter is coupled. The second waveguide 102 is designed to have a refractive index less than that of the first waveguide 100 while having a large mode size to obtain an advantageous input/output coupling with an optical fiber. The input terminal 106 has a waveguide constituted only by the second waveguide 102. This second waveguide 102 uses air as its upper clad while using the substrate 104 as its lower clad in order to confine optical waves in a depth direction. In order to confine optical waves in a longitudinal direction, the second waveguide 102, which serves as a core, is partially etched to have a rib waveguide structure.
The output terminal 108 has a waveguide constituted only by the first waveguide 100. The first waveguide 100 of the output terminal 108 has a strip loaded waveguide structure different from the rib waveguide structure of the input terminal 106. The first waveguide 100 uses air as its upper clad while using the second waveguide 102 as its lower clad.
A mode conversion region is defined between the input and output terminals 106 and 108 in order to convert a mode coupled after being inputted from the optical fiber to the optical device into a mode shape suitable for execution of the functions of the optical device without any loss of the coupled mode. The rib waveguide having a large mode size is converted into the strip loaded waveguide having a small mode size by the mode conversion region. A light guided through the mode shape converter is slowly shifted toward the first waveguide 100 because the first waveguide 100 has a refractive index higher than that of the second waveguide 102 even though the widths of both the first and second waveguides 100 and 102 increase. When the guided light reaches the output terminal 108, the power thereof is mainly concentrated toward the first waveguide 100.
FIG. 2a is a diagram illustrating a mode profile of the input terminal 106 in the above mentioned mode shape converter whereas FIG. 2b is a diagram illustrating a mode profile of the output terminal 108 in the mode shape converter.
However, the integrated optical device provided with the above mentioned mode shape converter has problems as follows. First, the fabrication is troublesome because it is necessary to use two cores made of different materials, and the first waveguide should be precisely formed on the second waveguide. Second, there is a limitation in minimizing the coupling loss of the optical device to an optical fiber having a circular mode because the input terminal 106 has a rib waveguide structure having an oval waveguide mode even though it has a large mode size. Third, since the mode shape converter uses a down-tapering structure in order to increase the mode size of the input-end waveguide, its waveguide taper increases in length. An increase in transmission loss occurs during the mode conversion.
Therefore, an object of the invention is to provide a mode shape converter including a double waveguide made of a single medium while having an up-tapering structure, a method for fabricating the mode shape converter, and an integrated optical device using the mode shape converter.
In accordance with one aspect, the present invention provides a mode shape converter interposed between an input or output terminal of a function executing unit included in an optical device and an optical fiber and adapted to couple a mode of the optical fiber with a mode of the input or output terminal of the function executing unit comprising: a substrate; a lower clad coated over the substrate, the lower clad having an etched portion in a desired region; a lower rib waveguide formed on the etched portion of the lower clad; a core formed over both the lower rib waveguide and a non-etched portion of the lower clad; an upper rib waveguide formed on the core in such a fashion that it is aligned with the lower rib waveguide, the upper rib waveguide having a desired shape; and an upper clad formed over both the upper rib waveguide and a portion of the core not covered with the upper rib waveguide.
In accordance with another aspect, the present invention provides a method for fabricating a mode shape converter interposed between an input or output terminal of a function executing unit included in an optical device and an optical fiber and adapted to couple a mode of the optical fiber with a mode of the input or output terminal of the function executing unit comprising: (a) coating a lower clad over a substrate; (b) patterning an etch mask on the lower clad, and etching the lower clad to a desired depth using the resultant pattern of the etch mask; (c) coating a core layer over the etched lower clad, thereby forming a lower rib waveguide and a core; (d) patterning another etch mask on the core, and etching the core using the resultant pattern of the another etch mask, thereby forming an upper rib waveguide; and (e) coating an upper clad on the core and the upper rib waveguide.
In accordance with another aspect, the present invention provides an integrated optical device including a function executing unit coupled to optical fibers at input and output terminals thereof, respectively, a first mode shape converter arranged at the input terminal of the function executing unit and adapted to convert an input optical fiber mode into a mode suitable for execution of desired functions of the optical device, and a second mode shape converter arranged at the output terminal of the function executing unit and adapted to convert a mode outputted from the function executing unit into an optical fiber mode, the output-end mode shape converter having an arrangement reverse to that of the first mode shape converter, wherein each of the first and second mode shape converter comprises: a substrate; a lower clad coated over the substrate, the lower clad having an etched portion in a desired region; a lower rib waveguide formed on the etched portion of the lower clad; a core formed over both the lower rib waveguide and a non-etched portion of the lower clad; an upper rib waveguide formed on the core in such a fashion that it is aligned with the lower rib waveguide, the upper rib waveguide having a desired shape; and an upper clad formed over is both the upper rib waveguide and a portion of the core not covered with the upper rib waveguide.